Power conversion apparatuses such as an in-vehicle inverter for a railway vehicle, and an in-vehicle DC-DC converter for an electric car, which perform power conversion by a switching operation of a power semiconductor element, have become widespread. The power semiconductor element is sealed with resin, and constitutes a power semiconductor module. The power semiconductor module is used in the power conversion apparatus.
Upon a switching operation of the power semiconductor element, switching current flows through the wires inside the power conversion apparatus. The switching operation of the power semiconductor element refers to switching of a power semiconductor element between a low-resistance on-state and a high-resistance off-state at short intervals of several nanoseconds to several tens of microseconds. The switching current refers to a current that flows through the power semiconductor element when the power semiconductor element is performing the switching operation, and whose value varies by several tens of amperes to tens of thousands of amperes within a time as short as several nanoseconds to several tens of microseconds.
Due to the presence of a parasitic inductance in a wire inside the power conversion apparatus, a voltage is generated in the wire when a switching current flows through the wire in accordance with the law of electromagnetism. This voltage is often referred to as “surge voltage”. As the time rate of change of the switching current is greater and the parasitic inductance in the wire is greater, a higher surge voltage is generated in the wire. The surge voltage generated in the wire is applied to a component inside the power conversion apparatus. When a higher surge voltage is generated in the wire, the excessive surge voltage is applied to the component inside the power conversion apparatus and may result in breakage of the component.
To address that problem, conventional measures have been taken to connect a snubber to a component inside the power conversion apparatus in order to reduce the surge voltage applied to the component. For example, Patent Literature 1 listed below has disclosed a conventional technique to connect a snubber constituted by a diode, a capacitor, and an inductor, to a power semiconductor element in order to reduce the surge voltage applied to the power semiconductor element.